It is known that an analog/digital converter converts an input quantity X into an output quantity Y. The transfer function of an analog/digital converter is, having regard to the quantization, a non-linear function of the input value. The output value varies quantum-wise. A quantum is the difference in output value between two numerically consecutive values.
Ideally, all the quanta are equal and ideally, the mean value of a quantization plateau is deduced from the input value through a linear function of the form Y=AX. Such an ideal transfer function is represented in FIG. 1. This figure lies in the input/output plane and represents the output value along the ordinate versus the input value plotted along the abscissa. The output includes, in this example of FIG. 1, eight levels from 0 to 7 in integer values or expressed in binary base mode from 000 to 111. The output value retains the same value when the input varies between two consecutive values representing the quantization spacing q. A straight line D represented by dashes joins the mid-points of each of the plateaux.
It is also known that through its very principle, such an ideal converter systematically introduces some errors termed the quantization error. Statistically, this error has an equal probability of taking absolute values lying between EQU -q/2 and +q/2.
The slope of the line joining the middles of the segments representing the output value as a function of the input value is termed the gain line. Actual converters are, however, subject to drifting which, on the one hand, causes the gain of the converter to vary and, on the other hand, gives rise to an offset in the value of the zero of the converter. The errors thus incurred are referred to as the gain error and offset error.
The value of the systematic error of the ideal converter as a function of the input value is represented in FIG. 2. This error is alternately positive and negative and varies in jumps. For this reason, closed-loop feedback for bringing the offset error and the gain error to 0 is not undertaken in practice.
This type of error being known, a remedy therefor has however been attempted.
Thus, the U.S. Pat. No. 4,947,168 MYERS describes an analog/digital converter in which two types of corrections are performed, these two types of corrections being performed in open loop. On the one hand, a correction is performed by adding a correction value, stored in a memory 49, to the output value by means of an adder 29. The correction value is stored at an address of the memory which depends on an output value given by a first part of the converter converting the most significant bits (MSB). Total conversion results from the addition of the most significant bits obtained at the output of a converter 17, of the least significant bits (LSB) obtained at the output of a low-significance converter 27 and of the correction value originating from the memory 49. The values originating from the memory 49 are periodically renewed during the idle conversion times.
As explained (column 7, line 39 and column 8, line 51), a calibration sequence includes an initialization step and a calibration step. The calibration step consists in generating known values by means of a slow but accurate reference digital analog converter 43. The difference between the actual value given by the reference converter 43 and the value measured at the output of the low-significance converter 27 is stored in the memory 49 at the address indicated by the high-significance converter 17.
The initialization step consists in setting the gain and offset values of a fast but inaccurate digital/analog converter 21 used to subtract from the analog input value the value converted on the most significant bits by the high-significance converter 17. This step also makes it possible to adjust the offset error of the low-significance converter 27.
As explained in column 8, lines 46-51, the calibration step consisting in replacing the values stored in the memory 49 can be repeated periodically during the system idle times whereas the initialization step is in principle performed only at the beginning of a conversion period.
It is moreover, known to those skilled in the art to carry out adjustments to the offset value and to the value of the gain through open-loop monitoring which makes it possible to calculate errors and adjust them, manually or according to automated sequences.
As explained above, the subject of the present invention is an analog/digital converter (ADC) in which the correction of gain and of offset is performed in closed loop during the idle conversion times. As compared with the prior art such as results from the MYERS patent, the invention has the advantage of using a smaller number of memories. It also has the advantage of closed-loop regulation, on open-loop systems. This is intended to mean that after a time of operation, the residual errors, here in gain and offset, are near-0 errors and result from outside disturbances alone, due, for example, to the ageing of the components or to temperature deviations. The invention is particularly advantageous in cases where several analog quantities are firstly treated according to predetermined processes so as, for example, to reduce the dynamic range of quantities which are to be input to the converters. Such a case arises, for example, in television video signal acquisition cards in which analog voltage values representing the values of red, green and blue are transformed in a known manner and are defined by standards as luminance Y and red DR and blue DB difference signals. The signals Y, DR and DB make it possible to reconstruct, in a known manner, grey levels for a black and white image or red, green and blue signals so as to construct, in a known manner, a colour image. In such a case, the accuracy of the closed-loop correction makes it possible to guarantee good accuracy of the gain adjustments for an analog matrix for transforming analog quantities, even in the case of small matrix coefficients such as those encountered in matrices for converting to colour space. Moreover, in the case of television video signals, there are no signals to be converted on the one hand between each line and on the other hand between each frame. These periods in which there are no signals to be converted, termed blanking periods, are therefore regularly present during the conversion of video signals.